1. Field Of The Invention
The present invention relates to a process for preparing an ion exchange membrane which can advantageously be employed to separate the anode and cathode compartments of an electrolytic cell, especially a chlor-alkali electrolytic cell.
More particularly, the present invention is concerned with a process for preparing an ion exchange membrane in which a membrane of a fluorinated resin, having pendant ion exchange precursor groups, thereon is contacted with an aqueous solution of a basic organic compound for a period of time sufficient to hydrolyze the precursor groups.
2. Discussion Of Related Art
The use of an ion exchange membrane of a fluorinated polymer, having carboxylate and/or sulfonate ion exchange groups, as a membrane for separating the anode and cathode compartments of an electrolytic cell, especially a chlor-alkali electrolytic cell, has been increasing year by year. In this field of application, it is desired that the ion exchange membrane exhibit low electrolytic cell voltage and high current efficiency, thereby enabling the electrolytic cell having the membrane incorporated therein to be stably operated with low electric power supply.
The above-mentioned ion exchange membrane of a fluorinated polymer having carboxylate and/or sulfonate ion exchange groups is conventionally manufactured by first molding into a film a fluorinated polymer, having ion exchange precursor groups and having thermoplastic properties, and then hydrolyzing the ion exchange precursor groups to thereby form carboxylate and/or sulfonate ion exchange groups. The customary method for hydrolyzing the ion exchange precursor groups comprises contacting the precursor groups with an aqueous solution of an alkali metal hydroxide or with a mixture of an aqueous solution of an alkali metal hydroxide and an organic solvent selected from an alcohol such as methanol, ethanol or propanol and a water soluble organic solvent such as dimethyl sulfoxide. The alcohol or the water soluble organic solvent is added in order to increase the hydrolysis rate of the ion exchange precursor groups.
In the art, it is well known that an ion exchange membrane is likely to swell, thereby forming wrinkles on the surface of the membrane, when it comes into contact with a cell electrolyte. This wrinkle formation is likely to be accompanied by problems, such as cell voltage increase attributed to the retention of evolved gas and/or electrolyte by the wrinkles, and such as pinhole formation and membrane tearing attributed to the rubbing of the wrinkled membrane against an electrode. To cope with these problems, proposals have been made in which an ion exchange membrane is pre-swollen prior to installation in the cell by immersing the membrane in a specific organic solvent or an aqueous solution of an organic solvent. For example, U.S. Pat. No. 4,595,476 discloses a process for preswelling an ion exchange membrane in which use is made of an aqueous solution containing an organic solvent such as diethylene glycol and triethylene glycol. Further, U.S. Pat. No. 4,376,030 discloses a process for pre-swelling an ion exchange membrane in which the membrane is pre-swollen at a temperature of from 20.degree. to 80.degree. C. using an aqueous solution containing an amine selected from primary, secondary and tertiary amines, most preferably triethanolamine, in an amount of from 2 to 60% by weight.
However, in any of the hitherto proposed preswelling processes, two separate steps, i.e. a hydrolysis step and a pre-swelling step, are inevitably involved. Consequently, all of the known pre-swelling processes have a drawback in that a complicated procedure is necessary.